The non-destructive examination of a test piece by recording an image produced by the differential absorption of ionizing radiation often results in a radiograph having over-exposed portions. Such over-exposure may be due to characteristics of the test piece, such as dimensional variations (e.g. large variations of thickness), or variations in the coefficients of absorption of the radiation used for different materials in the test piece (e.g. steel, plastics, weld material, wiring, biological tissue etc.). Further the emulsion may be accidentally over-exposed.
Such over-exposed regions may contain important information of which the derivation is desirable or necessary, but due to their excessively high density cannot be interpreted by conventional means. It therefore becomes desirable to transform the high density of blackening in these regions into a density range which will permit interpretation, so as to derive the previously hidden information, in a negatoscope, microscope or densitometer.